With the many continued advancements in communications technology, more and more devices are being introduced in both the consumer and commercial sectors with advanced communications capabilities. Additionally, advances in processing power and low-power consumption technologies, as well as advances in data coding techniques have led to the proliferation of wired and wireless communications capabilities on a more widespread basis.
For example, wired and wireless communication networks are now commonplace in many home and office environments. Such networks allow various heretofore independent devices to share data and other information to enhance productivity or simply to improve their convenience to the user. One such communication network that is gaining widespread popularity is an exemplary implementation of a wireless network such as that specified by the WiMedia-MBOA (Multiband OFDM Alliance). Other exemplary networks include the Bluetooth® communications network and various IEEE standards-based networks such as 802.11 and 802.16 communications networks.
Computing the distance of a target wireless device from a reference wireless device is called ranging. Ranging can be performed by measuring the received signal strength (RSS) or the time of arrival (time-of-arrival) of the signal propagated between the target and reference node. The ranging accuracy using the received signal strength depends on the accurate modeling of path-loss and the propagation channel environment. On the other hand, the ranging accuracy using time-of-arrival typically depends on the estimation accuracy of time of arrival, because electromagnetic waves propagate at approximately the speed of light and thus a small error (in the nanosecond scale) in time translates to larger error in distance. The estimation accuracy of time of arrival depends on the available signal bandwidth, and the accuracy and resolution of the sampling clock frequency. Therefore, ultra wideband (UWB) technology that occupies greater than 500 MHz bandwidth is suitable for ranging and provides centimeter-level accuracy. The proposed effort for IEEE 802.15.3a and WiMedia standardizes UWB technology to provide high-rate (>53.3 MB/s) wireless connectivity in wireless personal network (WPAN) and WiMedia has proposed two-way ranging (TWR) as an additional feature for UWB systems. Also, the specifications of IEEE 802.15.4a for low-rate (<250 KB/s) WPAN makes ranging mandatory.
Ranging using multiple reference devices can enable positioning. Computing the 2D/3D position of a target wireless device relative to a coordinate system commonly known to a set of reference devices is called positioning. One common positioning system is the Global Positioning System, or GPS. Geodesic positioning obtained by a GPS receiver requires synchronous signal from at least four satellites. Although, the coverage of GPS positioning is broad, it requires line-of-sight connectivity from the satellites that may not exist in certain geographic locations. Indoor coverage may suffer as well, such as in office building, shopping mall, warehouse, auditorium, indoor stadium, parking structure. Furthermore, GPS receivers are costly and provide only meter-level accuracy.
Another ranging technique is the local positioning systems (LPS). Local positioning systems can provide indoor positioning using an ad-hoc wireless sensor network. To provide different emerging applications of location awareness, the location of such sensors has to be accurate and automatically configurable. A host of applications can be envisioned using the LPS. For example: (i) LPS for public safety—placing a localizer along a trail to mark the route, locating fire fighters in a burning building, locating children lost in the mall or park, etc.; (ii) LPS for smart home applications—automatic door opening when the resident is in the vicinity, activating certain appliances or devices depending on resident location, timing adjustment of light, temperature and sound level per individual profile, finding personal item such as pets, keys, purse, luggage; (iii) LPS in inventory control—localizers for real-time inventory, differentiating the contents of one container from the others; locating a book in the library, a document file in a law office; (iv) LPS for health care—hospital staff, patients and assets tracking, simplified record keeping and workflow, raising an alert if a staff member had not check a particular patient, visitor tracking for security, automatic pop-up of patient record on a tablet PC for a doctor's visit; and (iv) LPS for intelligent vehicle highway system—placing localizers along the side of a road to use as guide posts, placing localizers in vehicles to provide local intelligence for safety and providing centimeter level accuracy as opposed to meter level accuracy using GPS. Thus, ubiquitous use of position awareness implies local positioning systems which are expected to be low-cost, low-power, small-size and have scaleable accuracy.
Two-way ranging between a pair of devices has been used in various applications, including wireless networks. In general, ranging accuracy of the time-of-flight-based method depends on the signal bandwidth used in the transactions. However, assuming an operating bandwidth of the receiver to be higher than the signal bandwidth, the rate of the sampling clock affects the timing accuracy of ranging transactions—the higher the rate of the sampling clock, the higher the ranging accuracy. This is due to the fact that sampling with a higher clock frequency results in a more accurate timing resolution. However, due to the difficulty of accurately synchronizing all devices in certain applications, two-way ranging accuracies can be somewhat diminished. For example, if the respective clocks of the devices participating in the measurement have relative offset between them, a certain amount of error will be introduced in the measurement.
One way to improve the accuracy is to increase the frequency of the clock. At higher frequencies, the clock periods are shorter and thus the maximum offset is smaller. The higher clock frequency also makes time resolutions finer, reducing uncertainties related to time quantization noise. For example, using 528 MHz sampling clock rate gives the finite ranging resolution of 56.8 cm. Typically, the overall offset is statistically smaller as well. However, it is not always possible, practical or desirable to increase the rate of the sampling clock. Higher clock frequency requires higher complexity and higher power consumption in the device.